Strip Shape Analysis At The Air Knife Of A Continuous Galvanizing Line

The thickness and uniformity of the Zinc coating could greatly affect the performance of galvanized sheet,which is an important quality evaluation index of the galvanized sheet.During the production process,the sheet metal will undergo warping deformation when passing through the three rollers in the zinc pot(Mainly referring to "C-type tilt"),and the Zinc coating cannot maintain the desired thickness and uniformity at the air knife.For this reason,the quality of galvanized sheet is affected seriously.In order to solve this problem,the blower sheet metal system and the roller sheet metal system are taken as the research object,and with the help of the finite element simulation technology,the related models are established respectively,and the plate shape at the air knife is studied deeply in this thesis.What's more,the correcting process and mechanism of the correcting roll to the strip shape are discussed detailed;and these would provide a certain theoretical basis and technical guidance for the control plate shape and improving the quality of Zinc coating.Specific work is as follows:(1)Evaluation of the effect of non-uniform cooling on the shape of strip steel at the air knife.The evaluation is done in two steps.1)A special model is established to analyze the cooling process of a length of strip steel(12m in length)by coupled thermo mechanical method.In the model,the strip steel is not moving but the cooling source(bellows)moving downward.The results showed that,the non-uniform cooling could slightly lead to "C-type tilt" on the same cross section of the strip,and the difference of displacement between midpoint and the two endpoints on the same cross section could reach to 0.2×10-2mm.2)Taking the vertical up-leg following the zinc pot as the research objects,meanwhile,ignoring the cooling effect of bellows,a rectangular distribution load(distance from the top of the strip 19m)is applied on the strip center,and the load would lead to 3mm "C-type tilt" on the center.The simulation results show that the displacement of strip at air knife is almost zero.Therefore,the effect of non-uniform cooling on the shape of sheet metal at the air knife is very small.(2)Taking the roller sheet metal system in the zinc pot as the research object,then the finite element model of elastoplastic contact is established and carried out by the finite element software MSC.Marc.The calculation results show that,1)The obvious "C-type tilt" would occur when the strip passes the sink roll.2)The correcting roll plays an obvious correction role on the strip with "C-type tilt",meanwhile,the correcting mechanism and function are almost the same on the different sections.3)The "C-type tilt" after the sink roll and correction of the correcting roll is mainly caused by the “roll function” of each roll,and the “roll function” would destroy or improve the stress distribution along the width of the strip.It may increase the non-uniform deformation of the strip,and that could increasethe "C-type tilt",otherwise,the opposite results would obtain.4)It is not suitable that the correcting roll is pushed forward too much.There is an optimal value for a given steel strip.(3)Evaluation of the effects of the main parameters(such as the correcting roll engagement,tension of the steel strip,the friction coefficient between the roll and the strip and the strip speed)on the strip shape of different material is studied by the above mentioned models.In order to reduce the computational workload,the orthogonal experiment method is applied to filter the related parameter,and the orthogonal test table with four factors and three levels is established.The order of magnitude for each influential factor and the optimal parameter combination of the horizontal values in the table are calculated.What's more,the related parameters are debugged and optimized by further study on this base,and the optimal parameters are obtained that a more flat shape of steel strip at the air knife can be obtained.With these optimal parameters,the related model is calculated and the results show that the difference of displacement between midpoint and the two endpoints on the same cross section is about 0.22?10-3mm,which is much smaller than the thickness of Zinc coating quality requirement.